Kinetics and mechanism of permanganate oxidation of iota- and lambda-carrageenan polysaccharides as sulfated carbohydrates in acid perchlorate solutions

The kinetics of oxidation of iota- and lambda-carrageenans by permanganate ion in acidic solutions have been investigated, spectrophotometrically. Kinetic evidences revealed the formation of short-lived Mn III and/or Mn IV transient species. The kinetic parameters have been evaluated and a mechanism consistent with the experimental results is discussed. [Display omitted] ► Elucidation of reaction mechanisms for the oxidation of carrageenans as sulfated polysaccharides by acidic permanganate. ► Novel synthesis of ketoacid derivatives of ICAR and LCAR as biopolymer precursors of wide applications by the oxidation process. ► Behavior of sulfated polysaccharides in aqueous acidic solutions. ► Linear free-energy relationship for oxidation of polysaccharides by acidic permanganate. The kinetics of oxidation of iota- and lambda-carrageenan as sulfated carbohydrates by permanganate ion in aqueous perchlorate solutions at a constant ionic strength of 2.0 mol dm −3 have been investigated spectrophotometrically. The pseudo-first-order plots were found to be of inverted S-shape throughout the entire courses of reactions. The initial rates were found to be relatively slow in the early stages, followed by an increase in the oxidation rates over longer time periods. The experimental observations showed first-order dependences in permanganate and fractional first-order kinetics with respect to both carrageenans concentration for both the induction and autoacceleration periods. The results obtained at various hydrogen ion concentrations showed that the oxidation processes in these redox systems are acid-catalyzed throughout the two stages of oxidation reactions. The added salts lead to the prediction that Mn III is the reactive species throughout the autoacceleration periods. Kinetic evidence for the formation of 1:1 intermediate complexes was revealed. The kinetic parameters have been evaluated and tentative reaction mechanisms in good agreement with the kinetic results are discussed.